Enhanced laser pumping by auxiliary luminescent centers that absorb and transfer normally wasted pump energy to the laser ion

ABSTRACT

Sensitizer impurities absorb a portion of pump radiation in a laser generator that is not absorbed by the activator impurity. The sensitizer transfers the otherwise wasted radiation to the activator impurity. Mn +2 , UO 2   +2  and Ce +3  are examples of sensitizer ions. The foregoing sensitizer ions may, in turn, be sensitized by other impurities, known as cosensitizers to further utilize pump radiation.

This invention relates generally to optical masers or lasers and morespecifically to optical masers having an improved pump utilizationefficiency.

Present day lasers are severely limited by low efficiencies in theutilization of pump radiation. Optical pumps, as for example xenon flashlamps, emit or radiate light energy over a relatively broad spectralregion. The impurities or activators responsible for laser action,particularly the rare earth ions, have a comparatively narrow absorptionband and have heretofore utilized only a small fraction of the pumpradiation. The low efficiency of pump radiation utilization necessitateslarge power supplies for flash lamps in order to achieve even pulsedoutputs from lasers. For continuous laser operation even more severedemands are created for high energy outputs from the pump source. Thepoor utilization of pump radiation not only limits the number of laserswhich may be continuously operated but also often requires operation atliquid nitrogen or helium temperature to obtain the continuous laseraction. Broadening the spectral region of pump radiation which isutilized in the laser action would be a significant advantage in theart. An improved utilization of the broad spectrum of radiation couldlead to the use of pump sources presently unsuitable for pumping. Laserimpurities or activators which have threshold levels that cannot be metby present pump sources could be provided with sufficient pump energy toattain laser action. Lasers which are presently capable of producingonly pulsed outputs could provide continuous outputs with higher pumpradiation utilization and lasers which must now employ very lowtemperatures for continuous action could be operated continuously nearerto room temperature. The improved efficiency of the pump utilization is,of course, in itself advantageous and desirable.

Accordingly, it is the primary object of this invention to provide foran improved utilization of pump radiation.

It is an object of this invention to provide laser systems or generatorswhich have significantly improved efficiencies.

An object of this invention is to provide materials and means forutilizing heretofore unused bands of pump radiation for laser action.

Further objects and advantages of the invention will become apparent asthe following description proceeds and features of novelty whichcharacterize the invention will be pointed out in particularity in theclaims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to theaccompanying drawings, in which:

FIG. 1 is a graphic illustration of Mn⁺² and Nd⁺³ emission andabsorption spectra;

FIG. 2 is a partially sectioned elevation of an optical maser generator;

FIG. 3 is a cross-sectional detailed view of a laser rod which, inaccordance with this invention, may contain an activator, sensitizer andcosensitizer; and

FIG. 4 is a cross-sectional detailed view of a glass clad laser rod inaccordance with this invention.

In accordance with our discoveries, the foregoing and other objects areachieved in a manner which may be generally described as sensitization.As noted heretofore, the activator or laser ions absorb only a portionor small band of the available energy radiated from the optical pumpsource and a much larger portion or band of the radiated energy iswasted. We have found that the normally wasted energy can be transferredto the activator or laser ions by an auxiliary luminescent center. Morespecifically, we employ sensitizers or auxiliary impurity centers, otherthan the activator or laser ions, to absorb energy in a region of thepump band not being absorbed by the laser ion and to transfer thatadditional absorbed energy to the activator impurity. To accomplishthis, the spectral characteristics of the sensitizer impurity and theactivator impurity must be appropriately related, as outlinedhereinafter.

There must be an overlap between the sensitizer emission spectrum(whether observed or not) and the activator absorption band. Thesensitizer must absorb in a spectral region where the activator does notabsorb, within, of course, the region of pump radiation, and thereshould be little or no interfering absorption between the sensitizer andactivator. The sensitizer should preferably have a short radiativelifetime.

The particular choice of a sensitizer is determined by the activatorimpurity to be employed. We have found, for example, that the Mn⁺² andUO₂ ⁺² ions are sensitizers for the laser or activator ions Nd⁺³, Ho⁺³,Eu⁺³, Pr⁺³, Sm⁺³, Tb⁺³, Dy⁺³, Yb⁺³ and Er⁺³, while the Ce⁺³ ion is asensitizer for the activators Eu⁺³, Tb⁺³, Nd⁺³, Sm⁺³, Gd⁺³, Dy⁺³, Ho⁺³,Er⁺³, Tm⁺³ and Yb⁺³. The designated or matched sensitizer-activatorpairs have appropriately related spectral characteristics. Thesensitizer impurities will absorb a portion of optical pump radiationheretofore wasted and will transfer the portion to either of its pairedactivator impurities.

Referring now to FIG. 1 in the way of a specific example, we havegraphically illustrated the emission and absorption bands of a phosphateglass (MgO.P₂ O₅) containing the sensitizer Mn⁺² and the same glasscontaining the activator Nd⁺³. It is apparent from FIG. 1 that Mn⁺²absorbs in a region where the Nd⁺³ does not absorb and that Mn⁺² emitsin a region where the Nd⁺³ absorbs. The absorption of Mn⁺² in the 2.0 to2.2 eV region, a region where the Nd⁺³ absorbs only a minorinterference. The Mn⁺² - Nd⁺³ pair has exhibited a utilization of pumpradiation heretofore wasted by laser rods containing only the activatorNd⁺³ in actual tests.

Referring now to FIG. 2, we have illustrated an laser generator,generally known in the art. An optical pump 10, which may be a xenonflashtube, and a laser rod 11 are arranged within an elliptical cylinderhousing 12 and located at the respective axes of the cylinder. Theinside surface 13 of the housing is a highly polished reflector. Thelaser rod 11, illustrated in detail in FIG. 3 has reflecting mirrors 14,15 at its ends, forming a resonator or cavity therebetween, the mirror14 being highly reflecting and the mirror 15 being highly reflecting andpartially transmitting. The laser rod 11 is made from an optical qualitysilicate glass having a base composition of 35 NK₂ 0.65 SiO₂, or abarium crown glass containing about 4 percent, by weight of Nd₂ O₃ andabout 4 percent, by weight of Mn⁺².

In operation, the flashtube 10 is pulsed to produce a surge of pumpingradiation which is directed to the laser rod. The Nd⁺³ ions in the roddirectly absorb a first narrow band of the broad pump radiation and theMn⁺² ions in the rod absorb another or second band of the broad pumpradiation. The second band of absorbed radiation is transferred by theMn⁺² ions to the Nd⁺³ ions. Both the energy directly absorbed by theNd⁺³ ions and that transferred from the Mn⁺² ions contributes to thenecessary pumping to create a population inversion and the laser action.The stimulated emission of radiation is amplified in the cavity and isemitted as the coherent beam 16 of FIG. 2.

Although it is preferred to employ the sensitizer impurity directly inthe host, together with the activator impurity for a more efficientenergy transfer, it is nonetheless possible to derive some increase inutilization of the pump radiation by, as illustrated in FIG. 4, claddingthe glass laser rod 20, which contains the Nd⁺³ ions, with a glass 21containing the sensitizer impurity. In the case of liquid lasers, forexample, the containing envelope could include the sensitizer.

It should be understood that the sensitizer must be present as the ion,in the indicated valence state, to effect the desired absorption andtransfer of energy. Any amount of the Mn⁺² ion should, theoretically,produce some of the desired absorption and transfer but at least about0.5%, by weight, of the glass host is considered an effective amount andfrom about 5 to 9 percent is preferred. The Mn⁺² ion may be introducedin the form of its carbonate or acetate together with the usual oxidesin the smelt mixture of the glass. Moreover, the sensitizer will beeffective in any glass host suitable for the laser action of the rareearth activator ions.

About 0.1 percent, by weight, of glass, is an effective amount of UO₂ ⁺²for sensitizing Nd⁺³ and Ho⁺³, up to about 6 percent may be included andabout 2 to 3 percent, by weight of glass host is preferred. About 0.5percent, by weight of glass, is an effective amount of Ce⁺³ forsensitizing Eu⁺³ and Tb⁺³ and about 5 to 9 percent is preferred. Itshould be understood that each of these sensitizers will theoreticallyproduce some improvement in utilizing pump radiation by transferringunused energy to the activator. These sensitizers may also be used inany glass host suitable for the laser action of the rare earth activatorion. UO₂ ⁺² may be introduced into the glass melt mixture as uranylnitrate, (UO₂) (NO₃)₂ and Ce⁺³ may be introduced in the form of itssulfate or chloride.

In accordance with principles known in the art, the rare earth activatorions are added to the glass host in amounts up to about 8 percent, byweight, as the oxide, e.g. Nd₂ O₃, Yb₂ O₃, Eu₂ O₃ and Tb₂ O₃. Theprinciple of sensitizer absorption and transfer of that absorbed energyis not dependent on the presence of any specific amount of activator inthe host. It should be emphasized that much smaller amounts than 8percent of the rare earth oxide will be sufficient to produce laseraction and that it is the rare earth ion which has the ability tofunction as a laser.

In accordance with another aspect of the invention, it has been foundthat three of the foregoing sensitizers, i.e. Mn⁺², UO₂ ⁺² and Ce⁺³ mayin turn be sensitized by other impurities, known hereinafter ascosensitizers, to even further improve the utilization of the broad bandof pump radiation. The relationship of the spectral characteristics ofthe cosensitizer and sensitizer is required to be the same as therelationship of spectra of sensitizer and activator. A summary ofexamples of suitable combinations is given in Table I, hereinbelow.

                  TABLE I                                                         ______________________________________                                        Activator                                                                             Sensitizer Cosensitizer                                               ______________________________________                                        Nd.sup.+3 Mn.sup.+2                                                                              As.sup.+3, Sb.sup.+3, Ce.sup.+3, Sn.sup.+3, Pb.sup.+2      Pr.sup.+3 Mn.sup.+2                                                                              As.sup.+3, Sb.sup.+3, Ce.sup.+3, Sn.sup.+3, Pb.sup.+2      Sm.sup.+3 Mn.sup.+2                                                                              As.sup.+3, Sb.sup.+3, Ce.sup.+3, Sn.sup.+3, Pb.sup.+2      Eu.sup.+3 Mn.sup.+2                                                                              As.sup.+3, Sb.sup.+3, Ce.sup.+3, Sn.sup.+3, Pb.sup.+2      Ho.sup.+3 Mn.sup.+2                                                                              As.sup.+3, Sb.sup.+3, Ce.sup.+3, Sn.sup.+3, Pb.sup.+2      Er.sup.+3 Mn.sup.+3                                                                              As.sup.+3, Sb.sup.+3, Ce.sup.+3, Sn.sup.+3, Pb.sup.+2      Yb.sup.+3 Mn.sup.+2                                                                              As.sup.+3, Sb.sup.+3, Ce.sup.+3, Sn.sup.+3, Pb.sup.+2      Tb.sup.+3 UO.sub.2.sup.+2                                                                        As.sup.+3, Sb.sup. +3, Ce.sup.+3, Sn.sup.+3,                                  Pb.sup.+2                                                  Dy.sup.+3 UO.sub.2.sup.+2                                                                        As.sup.+3, Sb.sup.+3, Ce.sup.+3, Sn.sup.+3, Pb.sup.+2      Tm.sup.+3 UO.sub.2.sup.+2                                                                        As.sup.+3, Sb.sup.+3, Ce.sup.+3, Sn.sup.+3, Pb.sup.+2      Nd.sup.+3 UO.sub.2.sup.+2                                                                        As.sup.+3, Sb.sup.+3, Ce.sup.+3, Sn.sup.+3, Pb.sup.+2      Pr.sup.+3 UO.sub.2.sup.+2                                                                        As.sup.+3, Sb.sup.+3, Ce.sup.+3, Sn.sup.+3, Pb.sup.+2      Sm.sup.+3 UO.sub.2.sup.+2                                                                        As.sup.+3, Sb.sup.+3, Ce.sup.+3, Sn.sup.+3, Pb.sup.+2      Eu.sup.+3 UO.sub.2.sup.+2                                                                        As.sup.+3, Sb.sup.+3, Ce.sup.+3, Sn.sup.+3, Pb.sup.+2      HO.sup.+3 UO.sub.2.sup.+2                                                                        As.sup.+3, Sb.sup.+3, Ce.sup.+3, Sn.sup.+3, Pb.sup.+2      Er.sup.+3 UO.sub. 2.sup.+2                                                                       As.sup.+3, Sb.sup.+3, Ce.sup.+3, Sn.sup.+ 3,                                  Pb.sup.+2                                                  Yb.sup.+3 UO.sub.2.sup.+2                                                                        As.sup.+3, Sb.sup.+ 3, Ce.sup.+ 3, Sn.sup.+3,                                 Pb.sup.+2                                                  Pr.sup.+3 Ce.sup.+3                                                                              Pb.sup.+2, Bi.sup.+3                                       Nd.sup.+3 Ce.sup.+3                                                                              Pb.sup.+2, Bi.sup.+3                                       Sm.sup.+3 Ce.sup.+3                                                                              Pb.sup.+2, Bi.sup.+3                                       Eu.sup.+3 Ce.sup.+3                                                                              Pb.sup.+2, Bi.sup.+3                                       Gd.sup.+3 Ce.sup.+3                                                                              Pb.sup.+2, Bi.sup.+3                                       Tb.sup.+3 Ce.sup.+3                                                                              Pb.sup.+2, Bi.sup.+3                                       Dy.sup.+3 Ce.sup.+3                                                                              Pb.sup.+2, Bi.sup.+3                                       Ho.sup.+3 Ce.sup.+3                                                                              Pb.sup.+2, Bi.sup.+3                                       Er.sup.+3 Ce.sup.+3                                                                              Pb.sup.+2, Bi.sup.+3                                       Tm.sup.+3 Ce.sup.+3                                                                              Pb.sup.+2, Bi.sup.+3                                       Yb.sup.+3 Ce.sup.+3                                                                              Pb.sup.+2, Bi.sup.+3                                       ______________________________________                                    

The cosensitizer will absorb energy in a spectral region where neitherthe activator nor sensitizer absorb and there should be little or nointerfering absorption between the cosensitizer and either thesensitizer or activator. Whether observed or not, there must be anoverlap between the cosensitizer emission spectrum and the sensitizerabsorption band. Again, there is no criticality as to concentration ofcosensitizer so long as it is present as the ion and in the indicatedvalence state, although a concentration from 0.5 to 8 percent by weightis preferred. The cosensitizer is preferably included in the glass hostlaser rod together with the sensitizer and activator but it may also beincluded in a glass cladding coating, similar to that illustrated inFIG. 4.

We claim as our invention:
 1. A laser medium comprising a host, a Nd⁺³activator ion capable of laser action in the host and a sensitizer ionin said medium capable of absorbing and transferring energy to theactivator ion, the sensitizer selected from the group of ions consistingof Mn⁺² and UO₂ ⁺².
 2. The laser medium of claim 1 wherein thesensitizer is selected from the group consisting of Mn⁺² and UO₂ ⁺² andthe medium contains a cosensitizer ion selected from the groupconsisting of As⁺³, Sb⁺³, Ce⁺³, Sn⁺³ and Pb⁺².
 3. The laser medium ofclaim 2 wherein the sensitizer is UO₂ ⁺².
 4. A glass host laser rodcontaining Nd⁺³ activator impurity ions capable of laser action in saidhost and sensitizer impurity ions capable of transferring energy to theactivator impurity ions, the sensitizer impurity ions selected from thegroup of ions consisting of Mn⁺² and UO₂ ⁺².
 5. The laser rod of claim 4wherein the glass host is selected from the group consisting of,silicate and barium crown glasses.
 6. The laser rod of claim 4 whereinthe glass host is a potassium silicate glass.
 7. The laser rod of claim4 wherein the sensitizer impurity ions are Mn⁺² in an amount from about0.5 to 9 percent by weight of the glass host.
 8. The laser rod of claim4 wherein the sensitizer impurity ions are UO₂ ⁺² in an amount fromabout 0.1 to 6 percent by weight of the glass host.
 9. The laser rod ofclaim 4 wherein the host also contains cosensitizer impurity ionsselected from the group consisting of As⁺³, Sb⁺³, Ce⁺³, Sn⁺³ and Pb⁺².10. In a laser generator comprising a resonant cavity, means forextracting coherent radiation from the cavity, an activator impurity ionwithin said cavity, the activator being capable of producing astimulated emission of radiation in response to an absorption of a firstrelatively narrow band of radiation from a relatively broad band ofradiation including said narrow band, the improvement comprisingsensitizing ion means within said generator adapted to absorb a portionof the radiation in a second band not substantially within said firstband and to transfer an absorbed portion to the activator impurity, thespectral characteristics of the activator ion impurity ion andsensitizing ion means providing an overlap between the sensitizeremission spectrum and the activator absorption band with essentially nointerfering absorption between said ions, and further includingcosensitizing ion means to absorb another portion of radiation in athird band not substantially within said first and second bands and totransfer the other portion to said ion means absorbing in the secondband, the spectral characteristics of the cosensitizing ions means andsensitizing ion means providing an overlap between the cosensitizing ionmeans emission spectrum and sensitizing ion means absorption band,whereby a greater portion of the radiation may be utilized by theactivator impurity.